It seems reasonable to believe that most Americans — the most notable exception being U.S. Sen. John McCain of Arizona — would prefer that the United States avoid becoming militarily involved in the Syrian civil war.

After paying the tremendous costs of fighting in Iraq and Afghanistan, the average American’s appetite for combat on foreign soil is surely at its lowest point since the Vietnam War. So Syria, although representing a serious problem the United States cannot ignore, isn’t likely to make McCain a successful campaigner for direct American aid to the Syrian rebels.

President Obama had said that if the Assad regime that has ruled Syria for more than 40 years resorted to the use of chemical weapons to put down the rebellion then he would regard that as Syria having crossed the line and that would require American intervention.

Recently, there were reports that Assad’s forces had employed chemical weapons, believed to be sarin (a deadly nerve gas developed during World War II), and so the president found himself being egged on by McCain (his rival in the 2008 presidential election) and others who favor the direct deployment of American resources against Assad.

But new questions have arisen over the source of the soil and other samples from Syria and there are apparent inconsistencies between eyewitness accounts describing one of the attacks and textbook descriptions of sarin. Arms control experts are sufficiently uncertain that the Assad regime has actually used chemicals that Obama is justifiably reluctant to intervene.

Last week, the White House did suggest that Syrian troops may have used the nerve gas in two attacks while rebel spokesmen have insisted there were actually four such attacks and that they cost 31 lives. But there’s still no proof, and the rebels, especially, can be fairly suspected of using the reports as a way to lure the United States into the conflict on their side.

Yet there is a serious argument that, from a global perspective, any use of chemical agents in warfare must be addressed firmly and aggressively.

“One of the few positive outcomes of World War I was the Geneva Protocol of 1925, in which world leaders agreed that they would no longer use chemical or biological weapons,” Max Fisher, who writes for The Washington Post, observed this past weekend.

And, he added, their agreement has largely worked.

“The civil war in Syria has already killed tens of thousands of people, and the regime has already been accused of killing large numbers of civilians, including children, so why does it matter if regime forces used chemical weapons in small amounts, as U.S. intelligence believes they may have?” Fisher wrote.

His answer: “It’s not absurd or inconsistent to suggest, as the Obama administration did when it declared a ‘red line’ against the use of chemical weapons on Syria, that killing even just a few people with chemical weapons is somehow different than killing lots of people with conventional weapons.”

But, he argued, it’s not just about Syria. It is also about every war that will follow and about the kind of warfare the world will accept. It’s also “about preserving the small but crucial gains we’ve made over the last century in constraining warfare in its most terrible forms,” Fisher concluded.

McCain is surely sincere in arguing for American intervention, and the world must oppose the use of chemical weapons, but intervention is risky. The president must weigh all the arguments before acting lest Syria’s war becomes a global rather than merely a regional tragedy.